1. Field of the Invention
The present invention relates generally to non-aqueous alkali metal/oxygen battery cells, and to electrolyte solvents that are beneficial to their performance. An exemplary embodiment of the instant invention is a non-aqueous lithium/oxygen battery cell comprising a protected Li anode, a non-aqueous electrolyte and molecular oxygen accessed from the ambient air environment.
2. Related Art
The low equivalent weight of alkali metals, such as lithium, renders them particularly attractive as a battery electrode component. For example, lithium metal is both lightweight and energetic. The faradaic capacity of lithuim is 3800 mAh/gr while it's electrochemical potential vs. SHE (standard hydrogen electrode) is −3.05 V. Lithium batteries are prevalent today: lithium primaries are commonplace (e.g., Li/MnO2, Li/FeS2, Li/SO2 and Li/SOCl2) and Li-Ion is the premier secondary battery for powering digital electronics. In no short measure, the incompatibility of Li anodes in most solvents was a major obstacle in the early development of what are now considered conventional Li batteries. Generally, Li anode stability hinges on the formation and integrity of a protective solid electrolyte interphase (SEI) layer that forms on the surface of the anode (e.g., Li metal or LiCx) in contact with battery electrolyte. Targeting an electrolyte composition to form a stabilizing SEI layer that impedes further reaction but does not significantly hinder battery performance is not a trivial task. Exhaustive experimentation is required, and alteration of the electrolyte formulation in any way or contamination by moisture will generally ruin the integrity of the layer and the functionality of the anode. Electrolyte preparation is also quite tricky, and great care and effort is needed to ensure that electrolyte composition (combination of salts and solvents) is properly formulated and “dry” (low moisture content); to this end manufactures expend significant resources purifying solvents and preventing moisture contamination during electrolyte preparation and battery assembly. Electrolyte moisture levels in conventional lithium batteries are typically kept below about 50 ppm. Moreover, there are only a limited number of solvents that are compatible with Li anodes, and this severely restricts electrolyte optimization for the cathode's benefit.
Conventional lithium batteries (both lithium metal primaries and Li-Ion rechargeable) are performing to near optimal levels, so enhancement in energy density is expected to improve only incrementally from this time forward. To meet the present demand for significant increases in battery operating life and energy density next generation lithium battery systems need to make use of lighter and more energetic cathode materials. The use of oxygen as an electrochemically active cathode reactant in a lithium battery can be highly advantageous. Oxygen like lithium has a very low equivalent weight, and its electrochemical potential vs. Li metal indicates that Li/O2 battery cells can provide a potential of about 3V with a theoretical energy density of about 5200 Wh/kg. What's more, O2 is available, “freely”, from the air, if it can be properly harnessed.
K M Abraham and Z. Jiang describe a Li/O2 polymer electrolyte battery in U.S. Pat. No. 5,510,209 filed on Jan. 5, 1995 and in a technical paper published shortly thereafter in the J. Electrochemical Soc., Vol 143, No. 1 Jan. 1996. The battery cell disclosed by Abraham and Jiang uses a polymer electrolyte containing organic liquid electrolyte (effectively a gel electrolyte) as a separator sandwiched between a lithium metal anode and an oxygen cathode. If exposed to air, the polymer electrolyte will not prevent moisture from reaching the lithium anode, as no polymers are impervious to moisture, particularly polymer electrolytes that are imbibed with hydroscopic liquid electrolyte (such as a PC/EC mixture with LiPF6). The battery cells disclosed by Abraham, if exposed to ambient air, are therefore susceptible to high rates of self-discharge and severe anode degradation associated with lithium corrosion by moisture. Furthermore, batteries that self-discharge also tend to have limited operating life and generally low capacity (mAh/cm2). Abraham also mentions in his technical paper that cathodes used in Li/O2 battery cells tend to form discharge products in the cathode that choke the pores and limit battery performance.
There is a pressing need for practical high energy density batteries to meet the present demands for increased battery life. Li/Air batteries have the potential to meet this demand, but are not yet practical due to self-discharge caused by anode corrosion with moisture from the air and poor cathode performance. Accordingly, there is an immediate need to enable practical high energy density Li/Air batteries having high delivered area capacity (mAh/cm2) with minimal self-discharge by protecting the anode and generally improving cathode performance.